Bioactive 13,28-oxidooleanane-type saponins from Measa kamerunensis Merk

Four previously undescribed 13β,28-oxidooleanane-type triterpenoid saponins, maekamerunosides A-D (1-4), together with one new triterpenoid aglycone, maekamerunogenin (6), and the known saponin maesargentoside IV (5), were isolated from the stem bark of Maesa kamerunensis. Their structures were elucidated by extensive spectroscopic analyses (1D and 2D NMR, HR-ESI-MS) and chemical methods, including acid hydrolysis and GC-MS analysis of sugar moieties. The isolated saponins share a common oleanane-type aglycone featuring a rare 13β,28-epoxy bridge and are characterized by complex oligosaccharide chains containing glucuronic acid, galactose, and rhamnose units. Biological evaluation of the MeOH extract, n-BuOH fraction, and isolated compounds (1-5) revealed moderate to significant bioactivities. Compound 4 exhibited the highest antioxidant activity with IC₅₀ values of 95.19 ± 1.15, 135.74 ± 0.80, and 117.04 ± 0.98 μM in β-carotene-linoleic acid, DPPH•, and ABTS^+• assays, respectively. Compound 5 showed notable butyrylcholinesterase inhibition (IC₅₀ = 93.4 ± 0.74± 0.74 μM) and the highest inhibitory effects against α-glucosidase (31.29 ± 0.85%) and α-amylase (27.72 ± 0.77 ± 0.77%) at 200 μg/mL. Additionally, compound 5 demonstrated the strongest urease inhibition (IC₅₀ = 15.90 ± 0.52 μM), while no significant tyrosinase inhibition was observed. These findings suggest that M. kamerunensis is a valuable source of structurally diverse triterpenoid saponins and that synergistic effect in the crude extract may contribute to its enhanced bioactivity.